Observations in Cascadia Basin on the eastern flank of the northern Juan de Fuca Ridge reveal significant bottom water modification as a result of regional conductive heating and local hydrothermal venting. Seafloor conductive heating occurs throughout the sedimented basin while hydrothermal fluid discharge is confined to small (~1 km2) isolated igneous basement outcrops. In the northern sector of the plateaulike basin, the vertical fluxes of heat and mass associated with these floor processes lead to the formation of a 250- to 350-m-thick ''geothermal boundary layer'' characterized by anomalously high temperature, reduced vertical stability, and high dissolved silicate concentration. Using a basinwide average lithospheric heat flux of 0.3 W m-2 and the observed thermal anomaly structure of the water column, we obtain a mean residence time of 1 to 2 years for the deep water over Cascadia Basin. Detailed water property data collected in 1992 and 1993 within the immediate vicinity of three isolated igneous basement outcrops in the north-central sector of the basin indicate that local bottom-water heating arises from low-temperature venting through the summit and flanks of the outcrops. Near the smallest edifice, especially well-defined layers of anomalously warm, particle-laden water were found within ¿20 m of the outcrop summit depth of 2610 m.

Maximum anomalies of temperature, light attenuation coefficient, and dissolved silicate concentration in the layers were 0.040 ¿C, 0.015 m-1, and 5 μmol L-1, respectively. We estimate the local heat flux, F'0, from the smallest outcrop to be (2.4¿0.8)U¿109 W, where U (m s-1) is the mean horizontal current at the venting depth. For reasonable mean currents in the range 10-3 to 10-2 m s-1, we find F'0≈0.2 to 2.4¿107 W. Assuming that the depressed conductive heat flow of -0.05 W m-2 observed through the sedimented seafloor surrounding the smallest outcrop reflects the advective loss of heat through the outcrop, the radial distance over which crustal fluids must collect heat and converge on the outcrop is about 10 km. ¿ American Geophysical Union 1995